1. Field of the Invention
The present invention relates to a method for manufacturing a photoelectric conversion device, typified by a solar cell or an image input sensor, which uses roll-to-roll system production means, and to a roll-to-roll system apparatus for manufacturing the photoelectric conversion device.
2. Description of the Related Art
As compared with a crystalline silicon material, an amorphous silicon film has such features that a film with a large area can be formed at a low temperature not higher than 400.degree. C., and a thickness of about 1 .mu.m is sufficient for the film as a photoelectric conversion layer to absorb light. Thus, the amorphous silicon film presents saving of silicon resources and lowering of manufacturing energy, and has hitherto attracted attention as a low cost material.
In a conventional technique, it has been common that a photoelectric conversion layer of a solar battery, an image sensor, a photosensor, or the like uses a diode structure in which a PIN junction is formed, so as to raise a photoelectric conversion efficiency or light responsibility. Although it is possible to form all of p-type and n-type semiconductor films and a substantially intrinsic i-type semiconductor film from amorphous silicon films, it is known that if a microcrystalline silicon material is used for the p-type and n-type semiconductor films, excellent photoelectric conversion characteristics can be obtained.
The reason is that in the photoelectric conversion layer of the PIN junction structure, since light absorption and generation of electric charges due to the light absorption occur in the i-type amorphous silicon film, the p-type and n-type semiconductor films are required to have high light transmissivity and to have such high conductivity that an excellent contact with an electrode can be achieved. For such requirements, the microcrystalline silicon film has properties of both low light absorption and high conductivity, so that it is a material suitable for the p-type layer and the n-type layer of the photoelectric conversion layer.
The amorphous silicon film is formed by a chemical deposition method (plasma CVD method) using a glow discharge plasma under a reduced pressure. The plasma CVD method uses a plasma CVD apparatus including a reaction chamber, exhaust means for keeping the reaction chamber under a reduced pressure, gas introduction means for introducing a raw material gas, means for generating a glow discharge plasma in the reaction chamber, and means for holding and heating a substrate. Although a silane (SiH.sub.4) gas is generally used as a raw material gas of an amorphous silicon film, it is also possible to use a disilane (Si.sub.2 H.sub.6) gas. Further, it is also possible to use a gas obtained by diluting the foregoing raw material gas with a hydrogen (H.sub.2) gas.
On the other hand, as a raw material gas of a microcrystalline silicon film, a mixture gas of a SiH.sub.4 gas and a H.sub.2 gas is used, and the film can be obtained when film formation is carried out under such a state that a diluting ratio of the H.sub.2 gas to the SiH.sub.4 gas has been raised. It is known that a microcrystalline silicon film itself to which a p-type or n-type conductivity determining impurity element is not added, shows n-type conductivity. However, in general, in order to control its conductivity of p-type or n-type and to raise its electrical conductivity, an impurity gas containing a p-type or n-type conductivity determining impurity element is added to the raw material gas and the film is manufactured.
In a technical field of semiconductor, an element of group III of the periodic table, typified by boron (B), aluminum (Al), gallium (Ga), and indium (In) is known as the p-type conductivity determining impurity element, and an element of group Vb of the periodic table, typified by phosphorus (P), arsenic (As), and antimony (Sb) is known as the n-type conductivity determining impurity element. In a general plasma CVD method, an impurity gas typified by B.sub.2 H.sub.6 or PH.sub.3 is mixed to the raw material gas and a film is formed. The addition amount of the mixed impurity gas at this time is about 0.1% to 5% of SiH.sub.4, and is not higher than 10% at most.
As described above, since the process temperature of the microcrystalline silicon film or amorphous silicon film is low, an organic resin film in addition to a glass material can be used as a substrate of a photoelectric conversion device. Since the thickness of several tens to several hundreds .mu.m suffices for the organic resin substrate and the organic resin film has flexibility, a roll-to-roll system production means can be applied. The roll-to-roll system can be applied to any step of the photoelectric conversion device, and it can be applied to a laser scribing step and a printing step for patterning, in addition to a step of forming an electrode by a sputtering method and a step of forming a photoelectric conversion layer by a plasma CVD method.
Basic steps relating to a photoelectric conversion layer of a solar battery, an image sensor, or the like manufactured on a substrate include a step of forming a first electrode on the substrate, a step of forming a photoelectric conversion layer which is made of a PIN junction and is in close contact with the first electrode, and a step of forming a second electrode which is in close contact with the photoelectric conversion layer. At the time of manufacturing the PIN junction which becomes the photoelectric conversion layer, the film formation is generally carried out without breaking a vacuum so as to improve the characteristics of a contact interface.
It is known that at this time, when the foregoing impurity gas is added to the foregoing raw material gas in order to form a p-type or n-type semiconductor film, a very small amount of impurity gas and its reaction product remain and are attached to a reaction chamber and a discharge electrode as a part of glow discharge plasma producing means.
For example, after the formation of an n-type silicon film, when a substantially intrinsic i-type amorphous silicon film is formed continuously in the same reaction chamber without adding an impurity gas, there has been a problem that the remaining impurity separates from the n-type silicon film and is newly taken in the i-type film. Since the substantially intrinsic i-type amorphous silicon film is manufactured such that the defect density in the film is made not higher than about 1.times.10.sup.16 /cm.sup.3, there has been a problem that even if the impurity element with a concentration of several tens ppm to several hundreds ppm is taken in, an impurity level is generated so that the characteristics of the film is changed.
In order to solve such problems, a multi-chamber separation type plasma CVD apparatus in which a plurality of reaction chambers are provided and gas separation means are provided between the respective reaction chambers, is put to practical use. In order to form the PIN junction, it has been necessary to provide at least three independent reaction chambers to form p-type, i-type and n-type semiconductor films.
Although the roll-to-roll system has a merit that productivity is excellent through a continuous process of an elongate substrate, various contrivances have been necessary to solve the foregoing problems in the plasma CVD apparatus. FIG. 2 is a schematic view of a conventional plasma CVD apparatus for a solar battery by a roll-to-roll system. An elongate substrate 212 wound on a bobbin 205 is disposed in a feeding chamber 210. The elongate substrate 212 fed from this chamber is rewound on a rewinding bobbin 206 provided in a rewinding chamber 211 through a coupling slit 209a, an n-type film forming chamber 201a, a coupling slit 209b, an i-type film forming chamber 201b, a coupling slit 209c, a p-type film forming chamber 201c, and a coupling slit 209d. At this time, the coupling slits 209a to 209d have functions to enable continuous movement of the elongate substrate 212 while a reduced pressure state is maintained between the respective film forming chambers 201a to 201c, and to prevent raw material gases and impurity gases supplied to the respective film forming chambers from diffusing and mixing with each other.
In order to make the elongate substrate smoothly pass through the coupling slit, the coupling slit with a wide cross section and a firm substrate supporting mechanism are required, while in order to prevent gases from mixing with each other, a narrow slit cross section and a mechanism for supporting and transferring the substrate without damaging the surface of a formed film are required. However, it has been very difficult to satisfy such inconsistent requirements. Especially, in the case where an organic resin film having flexibility is used, since the substrate itself is apt to deform, even if the coupling slit is precisely worked, the surface of a formed film has been often damaged.
Further, like other plasma CVD apparatuses, in the roll-to-roll system plasma CVD apparatus, it is necessary to at least provide gas introduction means 208 for introducing a SiH.sub.4 gas, an H.sub.2 gas, a B.sub.2 H.sub.6 gas, or a PH.sub.3 gas, exhaust means 202a to 202e, glow discharge plasma generating means 203a to 203c, and substrate heating means 204a to 204c, correspondingly to the respective reaction chambers, so that the structure of the apparatus has become complicated and large-scale. Thus, much labor has been required also in view of maintenance of the apparatus.